First, the atrophy rates in a composite volume measure of Alzheimer's disease-vulnerable regions (SPARE-AD) and the hippocampus demonstrated an APOE-ɛ4 gene-dose effect, with greatest atrophy among ɛ4 homozygous participants, followed by ɛ4 heterozygous participants, and least among ɛ4 non-carriers. The gene discussed is APOE; the disease is Alzheimer disease.